This invention relates to a method for maintaining consistency between a primary data base and a secondary image of the data base of a one on one redundant processor in a process control system, and more particularly, to a method for tracking changes to a primary data base and effecting a change to a secondary data base such that only predetermined areas changed are updated thereby achieving an increase in efficiency to perform the update function.
Process Control Systems with backup process controllers such as described and claimed in U.S. Pat. No. 4,133,027, issued to J. A. Hogan on Jan. 2, 1979, and U.S. Pat. No. 4,141,066, issued to Y. Keiles on Feb. 20, 1979, include a backup controller having a dedicated Random Access Memory (RAM) and a dedicated Read-Only Memory (ROM). The back-up controller is essentially idle or can be doing some background tasks, but not tasks relating directly to the process control function. Upon detection of a failure of one of the primary process controllers, the data stored in the RAM of the failed controller must be transferred to the RAM of the backup controller to perform the operations of the primary controller. These systems describe a 1:N redundancy system.
In the present invention there is provided a method, in a 1:1 redundancy system, whereby the data base of a secondary device (ie., secondary or backup controller) is updated periodically such that the updating process is transparent to the primary functions and does not tie-up (or penalize) CPU or processor performance and utilizes a minimum amount of time. The method of the present invention updates only the information which was changed, resulting in a more efficient use of the CPU or microprocessor, and allows the updating process to be performed more frequently, on a real-time basis, and makes it practical to track large quantities of data so that control dynamics are not affected on a failover. Thus, when a failover condition occurs, the time to get the secondary controller to take over for a failed primary controller is substantially reduced as well as being less of an impact to the process under control.
SUMMARY OF THE INVENTION
Therefore, there is provided by the present invention, a method for controlling the maintenance (ie. updating) of a data base in a redundant controller of a process control system. A process control system includes a process controller which has a first and second controller, one of the controllers being designated as a primary controller and the other controller being designated as a secondary controller. Each controller has an image of the data base, wherein the primary controller acts upon and updates the data base, and the secondary controller maintains an equivalent image of the data base. The primary controller performs predefined control functions of the process control system which includes updating the data base associated with the primary controller as a result of performing the predefined control functions. A method of the present invention for updating the data base associated with the primary controller comprises the steps of performing the control functions. Changes therefrom are utilized to update the data base associated with the secondary controller. Simultaneously with updating the data base, predetermined information being stored in the primary data base is collected. At the completion of performing the control functions, the predetermined information is transferred to the secondary controller, thereby updating the data base of the secondary controller.
Accordingly, it is an object of the present invention to provide a method for maintaining the data base of a redundant controller of a process control system.
It is another object of the present invention to provide a method for maintaining the data base of a redundant controller wherein only predetermined changes are updated.
It is still another object of the present invention to provide a method for maintaining the data base of a redundant controller of a process control system without significantly impacting CPU or microprocessor performance in the primary controller.